Process for increasing the strength of metallic bodies



Patented Oct. 13, 1925.

UNITED STATES HERMANN xlinnnnn, or nnnmm-rnmnnnnu, GERMANY, ASSIGNOR T0 SIEMENS} scnucxnnrwnnxn GESELLSCHAFFJ. MIT nnscnninxrnn HAFTUNG, or sIEMEns- STADT, 1mm BERLIN, GERMANY, a conronarron or GERMANY.

PROCESS FOR INCREASING 'I HE STRENGTH OF METALLIC BODIES.

No Drawing.

For many purposes 1n the engineering industry and in particular for the construction of motors, it is not sufiicient'to 'know the strength of the materials, such as lron and steel under tension, compression, bending stresses and the like, but it is also necessary to know their resistance to continuously changing stresses. In particularthe results of continuous shock, blow or impact tests (continuous shock bending tests) have attained particular importance in the arts. These tests permit, however,in com arison with the tensile strength tests not su ciently certain conclusions, parallel tests with similar test bodies of the same material frequently. showing widely deviating fracture values. It was therefore hitherto necessary to carry out a large number of parallel tests to obtain a fairly reliable result.

The present process is based upon the observation that the cause for the considerable differences in the results of the testsv is to be found in the fine cracks, fissures or scratches, which form on the surface of the test pieces during their preparation, par-.

ticularly in the bottomof the notches made in such test pieces. Such fissures invariably form the starting point of the fracture, although "they may be only the very finest scratches produced during polishing. 'According to the size and shape of these fissures, which may hardly be p'erceivable by the naked eye, sooner or later a permanent fracture is developed owing to the so-called notch action, the material at the surface being agaile and again strained beyond the elastic limit in consequence of the local crowding of stresses.

These drawbacks are considerably lessened if notentirely eliminated according :to this invention by attackin test body or at least 0 the notch bottom by the surface of the.

Application filed December 20, 1923. Serialflo. 681,849.

physical or chemical means, which are capable of removing all discontinuities in the surface structure, by seizing or attacking smaller particles than mechanical means, such as the highest grade tools, are capable of doing. If such means are employed the sharp fissures disappear and a dull but uniform and continuous surface is produced.

Such means are for instance emanations in which the particles may be ejected from the surface of the body itself (cathodeatOmization) or in which the surface may be bombarded by foreign particles (cathode rays).

Chemical means may also be employed,

for instance corroding agents, which dis- "solve .the surface with which they are brought in contact. 1 Heating up tothe fusing point of the material also attains-the end in view. The effect of'the heat must,

however, be confined to the most extreme outer layer, so that the structure of the lower layers is not changed or injured.

By employing all these various meansa particularly favorable circumstance aids the carrying out of the invention, viz, the fact, that all the forcesv which are capable to engage the small particles of the surface at all. operate automatically towards a removal of the unevennesses in thesurface. In regard to the cathode rays this fact .do'es not appear to call for a special explanation. All projecting portions are to a higher degree exposed to the impact of the bombarding particles and are quickly levelled by them, while recesses, cracksiand the like remain at first protected against the effect of thebombardment.

In the chemical action by dissolving'or corroding agents the same effect is attained. It is a well known fact, that a corrdding agent attacks prominent portions far more readily and removes them more speedily 5 than cavities, which are generally only broadened at the bottom. This also occurs under the minute conditions with which we are concerned here. A scratch or fissure in the surface first offers its edges or burrs to the corroding agent andonly gradually the action of the agent extends to the walls situated further 'in the interior and the bottom of the cavity is hardly attacked at all. The crack or fissure thus gradually disappears and its place is taken by a smooth concave recess or flute which has less of a the body are avoided.

notch or wedge action than the sharp fissure which it has displaced.

Test bodies of steel or other chemically composed substances are preferably treated not with one corroding liquid only, but with a number of different ones. These need not be mixed together, but may beevemployed alternately or successively. The additional corrosive agents are intended to dissolve the constituents which have not been sufficiently dissolved by the preceding agent.

The body to be treated may also be made the cathode of a vacuum tube. The cathode rays emanating from it then first level all projecting parts, such as the edges of cavities and cracks or scratches, so that here also dangerous discontinuities in thesurface and sudden changes in the cross-section of The fact that by fusing the most extreme outer surface layer by the application of temperatures up to the fusing point all sharp projections collapse and flow into the recesses or cavities can also be utilized for carrying out this invention.

The drawbacks mentioned in connection with the continuous shock bending tests,

which result from the traces left by the treatment of the surface of'the construe-- tion elements appear in conjunction with all kinds of stresses which are, liable to lead to permanent fractures, in, particular in the case of oscillation stresses of all kinds. Thus in all tests aiming at the determination of the permanent strength the possibility is given to eliminate the discrepancies which otherwise exist in the 'results of parallel tests.

It is not claimed as new to remove coarse cracks or fissures, which have obviously increased owing to continuous strains and have been recognized as starting points of 'an incipient permanent destruction, by milling or otherwise mechanically cutting the material away, so that a hollow or concave flute or'recess bead or channel is formed.

The invention is based upon the recognition; that not only such cracks which may be traced easily by the naked eye become gradually enlarged and finally cause a per manent fracture, but that discontinuities even of microscopic minuteness form the starting point for permanent fractures. As

no mechanical tool is capableof producing a surface, which is free from such fine irregularities or flaws, physical or chemical agents are according to this invention utilized, which are capable of dealing with such minute particles of the surface struc ture which are beyond the reach of mechanical tools. The means employed according to this inventionoperate essentially in the same manner in conjunction with very fine cracks or fissures as the mechani cal tool in conjunction-with coarse cracks or rents.

The process is particularly useful for producing test pieces of materials, but may also be employedfor treating parts of maor starting point of the permanent destruction. What I claim as my invention and dcsire-to secure by Letters Patent is 1. A process of increasing the strength of a metallic body, which consists in removing discontinuities in its surface by means,

capable. of attacking smaller particles of the surface structure than can beremoved by mechanical means.

2l-A process of increasing the strength I of a metallic"body, which consists in removing -'discontinuities in its surface by chemical means capable of attacking smaller particles of the surface structure than can be removed by mechanical means.

3. A process of increasing the strength. of. a metallic body, which consists in subject ing the surface of the body successively to the action of different corrodin'g agents to remove discontinuities in the body surface be 0nd the reach of mechanical tools.

11 testimony whereof I afiix my signature.

HERMANN KKNDLER. 

